1. Field of the Invention
This invention relates to a ceramic material or composition for making a honeycomb structure, such as a device for purifying the exhaust gas of an automobile, a filter for collecting fine particles from the exhaust gas of a diesel engine or a rotary regenerator type ceramic heat exchanger. More particularly, it relates to a ceramic composition which consists mainly of cordierite and LAS (lithium aluminum silicate), and which is useful for joining or bonding two or more ceramic structures of honeycomb construction together or a ceramic structure of honeycomb construction to another ceramic structure of different construction, or for covering or impregnating a ceramic structure of honeycomb construction to improve its strength and gas tightness.
2. Description of the Prior Art
There is known a ceramic structure of honeycomb construction which is widely used as, for example, a support, a filter or a heat exchanger. There is, for example, known a rotary regenerator type ceramic heat exchanger which comprises a cylindrical matrix of the honeycomb construction having a diameter of 30 cm to 2 m, and a matrix-holding ring fitting on the outer peripheral edge of the cylindrical matrix. This heat exchanger is divided into two halves by longitudinally extending means defining a heating fluid passage on one side thereof and a heat recovering fluid passage on the other side thereof. One of the halves is heated by a hot fluid and stores or absorbs heat, while the other half releases or transfers heat to a fluid to be heated. As the heat exchanger is rotated, each half thereof is alternately heated and cooled to perform heat exchange.
The rotary regenerator type ceramic heat exchanger as hereinabove described is required to have a high heat exchange efficiency, permit a fluid to flow therethrough smoothly without a substantial drop in pressure, and have a sufficiently high degree of thermal stress resistance to withstand a large temperature gradient. It is also important that the matrix is sufficently leakproof to maintain a high heat exchange efficiency and thereby a high operating efficiency of the system in which the heat exchanger is employed.
A large rotary regenerator type ceramic heat exchanger has hitherto been produced by a method which is disclosed, for example, in U.S. Pat. Nos. 4,303,585 and 4,357,987 which correspond to Japanese Laid-Open Patent Publication No. 46338/1980. According to this method, a plurality of molded segments for a honeycomb matrix are fired and bonded together by a ceramic binding material having substantially the same mineral composition as the matrix segments and differing therefrom only to a very small extent in coefficient of thermal expansion, and the bonded segments are fired. When the bonded segments are fired, however, the degree of thermal contraction of the binder differs from the degree of thermal expansion of the matrix segments. This difference brings about a reduction in the bonding strength of the joint and, as a result, their destruction due to thermal stress. This is particularly a big problem to a centrally supported rotary regenerator type ceramic heat exchanger. It has in its center a ceramic hub having an axial bore in which a rotary supporting shaft is fitted. The hub remains relatively cool, since it is not exposed to a hot fluid, but held in contact with the supporting shaft made of a metal and having good thermal conductivity, while the matrix segments are exposed to the hot fluid. Therefore, the joint between the hub and the matrix segments is not satisfactory in bonding strength.
U.S. Pat. No. 3,634,111, which corresponds to Japanese Patent Publication No. 14838/1972, proposes the use of a foamable lithium aluminum silicate (LAS) cement as a binder for a ceramic heat exchanger. U.S. Pat. No. 3,715,196, which corresponds to Japanese Laid-Open Patent Publication No. 5835/1972, proposes the use of a mixture of thermally devitrifiable glass and a fibrous flux which can be heat-treated to form crystalline glass. These binders, however, also have a number of drawbacks. The former binder is not satisfactory in chemical stability to acids and sodium. Moreover, its softening temperature not exceeding 1300.degree. C. limits the scope of its application. The latter is unsatisfactory in heat resistance and thermal expansion characteristics and, therefore, has only a limited scope of application.
Various methods have been proposed for reducing the leakage of a fluid from a rotary regenerator type ceramic heat exchanger. The leakage can be reduced if the leakage of a fluid through the pores of a partition in a honeycomb matrix or through the layers joining the matrix segments is reduced. If the heat exchanger is in the shape of a ring, the leakage can be reduced by the provision of a leakproofing coating layer on each of its inner and outer peripheries. The use of a highly gastight ceramic binding material enables the layers joining the segments to reduce the fluid leakage. A drastic reduction in leakage can be expected if a highly gastight coating is applied to the outer periphery of the heat exchanger, or if the pores of the partition in the matrix are sealed by a highly gastight material.
The matrix of a honeycomb structure is, however, usually formed of a material having a low coefficient of thermal expansion, such as LAS, cordierite or aluminum titanate. The joining of the matrix segments, the coating of the outer periphery of the matrix and the sealing of the pores of its partition must, therefore, be performed by a material having a low coefficient of thermal expansion which differs only to a small extent from that of the matrix material. No such material has hitherto been discovered.
The use of a low thermal expansion type composite material composed of cordierite and LAS has been extensively studied, but has not yet given any satisfactory result. On the contrary, it has hitherto not been considered applicable to a heat exchanger, since its coefficient of thermal expansion is increased by the formation of a crystalline phase, such as of mullite, spinel or corundum, when it is fired.